Electron periodicities in Saturn's outer magnetosphere

摘要

The Magnetospheric Imaging Instrument/Low-Energy Magnetospheric Measurements Sensor (MIMI/LEMMS) instrument on the Cassini spacecraft has measured energetic electrons in the energy range 20–300 keV within Saturn's magnetosphere. In the outer magnetosphere, beyond ～20 R S , these electrons and their spectral index display strong variations with periods comparable to the 10.76 hour (10 hours, 45 min, 36 s) period measured by radio observations of Cassini. Inside ～20 R S , such electron variations may be present but are masked by satellite and ring effects. Electron periodicities are most easily recognized on the “nightside” segments of the Cassini orbits, although they are also observed to some extent on the dayside. For both dayside and nightside a wavelet analysis of detrended count rates in the 20–40 R S region reveals a mean period of 10.52 ± 0.74 hours (10 hours, 31 min ± 44 min) for the six electron channels investigated. If constrained to the nightside only, a wavelet analysis gives a mean period of 10.88 ± 0.52 hours (10 hours, 53 min ± 31 min). Covering five orbits of the Cassini spacecraft during the 2-year period from Saturn Orbit Insertion (July 2004 to July 2006), this analysis indicates that a day-night asymmetry exists in the electron periodicity in Saturn's outer magnetosphere and suggests that dayside periodicities are much less prominent than nightside or dawnside periodicities.